3. DWARF GALAXIES

Gas-rich dwarf galaxies and in particular Blue Compact Dwarfs (BCDs)
were originally expected to have their FIR emission dominated by dust
heated locally in HII regions. Temperatures of 30K or more were
anticipated. This was the a priori expectation in particular for the
BCDs, and became the standard interpretation for the IRAS results
obtained for these systems. Hoffman et al.
[44],
Helou et al.
[37]
and Melisse & Israel
[57]
each found that the 60/100 µm colours of BCDs were clearly
warmer than those of spirals.

The IVCD Survey (Tuffs et al.
[88],
[89])
changed that simple picture of
the FIR emission from dwarf galaxies. The IVCDS included measurements
at 60, 100, and (for the first time) at 170 µm of 25 optically
selected gas-rich dwarf galaxies. The observations at 60 and
100 µm were consistent with the previous IRAS results,
though extending knowledge of these systems to
lower luminosities. Unexpectedly however, high ratios of
170/100 µm luminosities were found in many of the surveyed
systems. Such long-wavelength excesses were found both in relatively
high-luminosity dwarfs, such as VCC655, as well as in fainter objects
near the limiting sensitivity of the survey. These observations imply
the presence of large amounts of cold dust.

As shown by Popescu et al.
[73],
it seems unlikely that the cold
dust resides in the optically thick molecular component associated
with star-formation regions, since the implied dust masses
would be up to an order of magnitude greater than those typically
found in giant spirals. Such large masses could not have been
produced through star-formation within the dwarfs over their
lifetime. One alternative is that the dust
originates and still resides outside the optical extent of these
galaxies. In fact, some evidence for this was provided by the ISOPHOT
observations themselves, since they were made in the form of
scan maps, from which estimates of source sizes could be
determined. Even with the relatively coarse beam (1.6' FWHM at
170 µm), the extended nature of the sources could be clearly
seen in a few cases
for which the FWHM for the 170 µm emission exceeds the
optical diameters of the galaxies (to 25.5 Bmag/arcsec2) by
factors of between 1.5 and 3.5. It is interesting to note that two of these
galaxies (VCC 848 and VCC 81) have also been mapped in HI (Hoffman et
al. [45]),
revealing neutral hydrogen sizes comparable to the
170 µm extent. This raises the possibility that the
cold dust is embedded in the extended HI gas, external to the optical
galaxy. This would be analogous to the case of the edge-on spiral
NGC 891, where Popescu & Tuffs
[68]
discovered a cold-dust counterpart to the extended HI disk (see
Sect. 2.1.2). In
this context the main observational difference between the giant
spiral and the dwarfs may be that for the dwarfs the integrated
170 µm emission is dominated by the extended emission component
external to the main optical body of the galaxy, whereas for the giant
spirals the long-wavelength emission predominantly arises from within
the confines of the optical disk of the galaxy.

Apart from the SMC (which is discussed in
Sect. 3.2 and is
too extended for ISOPHOT to map beyond its optical
extent), only three dwarf galaxies were observed by ISOPHOT in the
field environment (all Serendipity Survey sources; Stickel et
al. [81]).
All three sources have comparable flux densities at 100 and
170 µm. But the small statistics mean that it is still an open
question to which extent the cold dust emission associated with the
extended HI component in dwarf galaxies is a cluster phenomenon or
not. Of potential relevance to the
origin of the dust seen surrounding Virgo dwarfs is the discovery by
ISOPHOT of the source "M 86-FIR" in the halo of M 86 (Stickel et al.
[80])
which has no optical counterpart. It may be a "relic" ISM
stripped from a spiral galaxy in the Virgo Cluster.

The existence of large quantities of dust surrounding gas-rich dwarf
galaxies may have important implications for our understanding of the
distant Universe. According to the hierarchical galaxy formation
scenarios, gas-rich dwarf galaxies should prevail at the earliest
epochs. We would then expect these same galaxies to make a higher
contribution to the total FIR output in the early Universe, certainly
more than previously expected.